1. Field of the Invention
The present invention relates to a terminal multiplexer which performs multiplexing and demultiplexing of digital signals between a plurality of low speed transmission lines and a high speed transmission line in a digital communication network and relates to a terminal multiplexer such as a channel rearrange equipment, for example ADM (Add Drop Multiplexer), having a cross connect function.
2. Description of Related Art
As a transmission system using a terminal multiplexer which performs multiplex conversion of signals between a plurality of low speed transmission lines and a high speed transmission line, there is known a system in which line terminating equipments (hereinafter, referred to as "LTE") are connected in a point-to-point manner, perform time division multiplexing of low speed signals received from a plurality of low speed transmission lines to send them as high speed signals onto a high speed transmission line, and perform demultiplexing of high speed signals received from the high speed transmission line into a plurality of low speed signals to send the demultiplexer signals onto respective low speed transmission lines, as shown in FIG. 13A.
As a configuration for realizing automatic protection switching in a transmission system using such LTEs, there are known 1:1 configuration and 1:n configuration. In the 1:1 configuration, a set of two working high speed transmission lines which transmit signals in opposite directions to each other and a set of two protection (i.e., preparatory) high speed transmission lines which transmit signals in opposite directions to each other are provided between the LTEs at both ends. In the 1:n configuration, between the LTEs at both ends, there are provided a plurality of working high speed transmission lines in sets of pairs, which transmit signals in the opposite directions to each other, and a set of two protection high speed transmission lines which an used in common with the plurality of high speed transmission lines and transmit signals in opposite directions to each other.
In the present description, working lines will be represented by the symbol "W" (working), and protection lines will be represented by the symbol "P" (Protection). Further, as for terminal multiplexer connected between two other terminal multiplexers, one side of the two other terminal multiplexers will be described as "West" and the other side will be described as "East".
Now, in LTEs in 1:1 and 1:n configurations, when a problem arises in a working high speed transmission line, automatic protection switching is performed so that the high speed transmission line used for transmitting signals is switched from the faulty one to a protection high speed transmission line. As a method for carrying out this switching, the bi-directional switching method and the uni-directional switching method are known. In the bi-directional switching method, as shown in FIG. 13B, both of the two high speed transmission lines in the faulty set are switched to two protection high speed transmission lines. In the uni-directional switching method, as shown in FIG. 13C, only the faulty high speed transmission line is switched to a line having the same direction of transmitting signals as the faulty line out of the protection high speed transmission lines.
Further, as a terminal multiplexer which performs multiple conversion between a plurality of low speed transmission lines and a high speed transmission line, as shown in FIG. 14A, there is known an ADM which performs demultiplexing of some high speed signals received from the high speed transmission line (West side) into a plurality of low speed signals to send them onto the respective low speed transmission lines, and performs time division multiplexing of the remaining high speed signals received from the high speed transmission line and low speed signals received from the low speed transmission lines to send the multiplexed signals onto the other high speed transmission line (East side), or performs similar operations in the reverse direction from the East side to the West side.
As a configuration of a transmission system using such an ADM, there are known a linear configuration in which ADMs are located between LTEs as shown in FIG. 14B, and a ring configuration in which a plurality of ADMs are connected in a ring shape with high speed transmission lines as shown in FIG. 14C.
As the linear configuration using ADMs, there are known two configurations corresponding respectively to the above-described 1:1 and 1:n configurations of LTFs. In the 1:1 linear configuration, as shown in FIG. 15A, on each of the West and East sides, there are provided a set of two working high speed transmission lines for transmitting signals in opposite directions to each other and a set of two protection high speed transmission lines for transmitting signals in opposite directions to each other. In the 1:n linear configuration, as shown in FIG. 15B, on each of the West and East sides, there are provided a plurality of working high speed transmission lines in sets of two, transmitting signals in opposite directions to each other, and a set of two protection high speed transmission lines which are used in common with the plurality of high speed transmission lines and transmit signals in opposite directions to each other. Here, at the time of the automatic protection switching of ADM in 1:1 and 1:n linear configurations, switching from the working high speed transmission lines to the protection high speed transmission lines is performed on each of the West and East sides, similarly to the above-described switching in LTE in 1:1 and 1:n configurations.
On the other hand, as the ring configuration in which ADMs are connected in a ring shape, there have been proposed a 2 fiber configuration in which each pair of adjacent ADMs are connected with a set of two optical fiber transmission lines transmitting signals in opposite directions to each other, and a 4 fiber configuration in which each pair of adjacent ADMs are connected with two sets of two optical fiber transmission lines transmitting signals in opposite directions to each other.
Further, as the 4 fiber ring configuration, there is known 4-Fiber BLSR (Bi-directional Line Switched Ring) in which, as shown in FIG. 16A, out of two sets of optical fiber transmission lines connecting each pair of ADMs, one set is used as working lines and the other set is used as protection lines. As the 2 fiber ring configuration as shown in FIG. 17A, there are known 2-Fiber UPSR (Uni-directional Path Switched Ring) and 2-Fiber BLSR. In 2-Fiber UPSR, optical fiber transmission lines transmitting signals in one rotational direction are used as working lines and optical fiber transmission lines transmitting signals in the other rotational direction are used as protection lines, and switching is performed for each path. In 2-Fiber BLSR, instead of setting a working or protection line for each optical fiber transmission line, some time slots on each optical fiber transmission line are used as working slots and the other time slots are used as protection slots.
Now, switching from working lines to protection lines at the time of automatic protection switching in 4-Fiber BLSR is illustrated in FIGS. 16B and 16C. As shown, as the switching performed by ADMs adjacent to a faulty portion in 4-Fiber BLSR, there are two kinds of switching, i.e., (1) switching from a set of working optical fiber transmission lines on the side of the faulty portion to a set of protection optical fiber transmission lines on the side of the faulty portion (FIG. 16B), and (2) turning back of signal flows from a set of working optical transmission lines on the opposite side to the faulty portion to a set of protection optical fiber transmission lines on the opposite side to the faulty portion (FIG. 16C), the former being called Span Switch, and the later Ring Switch.
FIG. 17B illustrates the switching from the working time slots to the protection time slots at the time of automatic protection switching in 2-Fiber BLSR, and FIG. 17C illustrates the switcing from the working path to the protection path at the time of automatic protection switching in 2-Fiber UPSR.
As shown, switching in 2-Fiber BLSR is performed in such a manner that ADMs adjacent to a faulty portion turn back signal flow in working time slots of two optical fiber transmission lines on the opposite side to the faulty portion into protection time slots of two optical fiber transmission on the opposite side to the faulty portion. In FIG. 17B, in the case that time slots A-F of #1 optical fiber transmission lines and time slots A-F of #2 optical fiber transmission lines are used as working time slots, and time slots G-L of #1 optical fiber transmission lines and time slots G-L of #2 optical fiber transmission lines are used as protection time slots, ADM A, B adjacent to the faulty portion turn back signal flow in the time slots A-F of #1 optical fiber transmission lines into the time slots G-F of #2 optical fiber transmission lines, and turn back signal flow in the time slots A-F of #2 optical fiber transmission lines into the time slots G-F of #1 optical fiber transmission lines.
Further, switching of 2-Fiber UPSR is performed as shown in FIG. 17C. Namely, each ADM transmits signals to other ADMs, using both the working optical fiber transmission lines and protection optical fiber transmission lines. In a normal condition, each ADM receives signals from other ADMs through working optical fiber transmission line and processes them, and when it can not receive from a particular ADM through the working optical fiber transmission line, it receives signals from that particular ADM through protection optical fiber transmission line and processes them.
As described above, functions required for a terminal multiplexer vary according to LTE used in the 1:1 configuration, LTE used in the 1:n configuration, ADM used in the 1:1 linear configuration, ADM used in the 1:n linear configuration, ADM used in 4-Fiber BLSR, ADM used in 2-Fiber BLSR, and ADM used in 2-Fiber UPSR. Accordingly, LTEs or ADMs have, conventionally, been made as dedicated equipments for each particular configuration of transmission system.
Sometimes, it is desired to change a configuration of a transmission system, for example, in order to make the transmission system advance after the start of its operation. For example, it may be desired that, in order to increase transmission capacity, a transmission system using LTEs connected in 1:1 configuration in a point-to-point manner is changed to a transmission system using LTEs connected in 1:n configuration in a point-to-point manner, or that a transmission system using LTEs connected in 1:1 configuration in a point-to-point manner is changed to 2-Fiber BLSR or 4-Fiber BLSR, in accordance with increase in connected points.
Conventionally, however, each LTE or ADM is a dedicated equipment for a transmission system before change, and therefore, when configuration of such a transmission system is to be changed, LTEs or ADMs should be exchanged, so that the burden of introducing equipments is large at the time of changing the configuration of a transmission system. Further, when LTEs or ADMs are exchanged, a transmission system must be taken down once, and communication must be stopped.
On the other hand, in accordance with recent increase in transmission capacity of a transmission system, a multiplex conversion equipment becomes of large scale. Accordingly, for example, it is, now, difficult to construct a terminal multiplexer adapted for OC-192 (optical carrier 192) using an optical fiber transmission line with 10 G of transmission capacity as a high speed transmission line, in one rack. Here, a "rack" is a case which houses electronic boards constituting a terminal multiplexer, and is provided with printed circuits connecting between electronic boards. A rack is limited in its size from the viewpoint of handling requirements such as transportation and installation. Thus, when a terminal multiplexer can not be constructed with single rack but with a plurality of racks, signals should be sent and received among the racks. To send and receive signals among the racks, cables should be used instead of printed circuit on an electronic board. Accordingly, and for other reasons, there are some limitations in terms of number and speed of signals, differently from sending and receipt of signals within a rack. Thus, for example, it is difficult to employ such a configuration that, in LTE etc. of 1:n configuration, n working high speed transmission lines and one protection high speed transmission line are connected to inputs of a single selector, and that selector switches the above-described working and protection lines.